The present invention relates to a method and apparatus for determining the presence or absence of a pour point depressant additive, also known as a wax crystal modifier and a flow improver additive, in hydrocarbon liquids derived from petroleum. The present invention relates to the novel discovery that the time-duration of a plateau for a hydrocarbon liquid cooling curve is indicative of the presence or absence of a pour point depressant. Thus, the presence or absence of a pour point depressant in samples of hydrocarbon liquids can now be readily identified.
The cloud point temperature of a hydrocarbon liquid derived from petroleum is a wellrecognized term in the petroleum industry and is further defined by ASTMD 2500, Standard Test Method for Cloud Point of Petroleum Oils. There has thus previously been references relating to cloud point determinations such as U.S. Pat. Nos. 3,031,880, 3,187,557, 3,457,772 and 3,545,254. Each of these references generally measure the cloud point temperature by measuring light passing through a sample while the sample is being cooled.
U.S. Pat. No. 3,447,358 cools a sample in such a way that convection currents are generated in the sample. At the cloud point, a crystal lattice formation reduces or stops the convection currents and this lowering or cessation of currents can be measured optically, mechanically, or acoustically.
Thus, it is known in the art that the cloud point temperature of an oil can be determined from a cooling curve (temperature versus time) of a hydrocarbon liquid sample which is based upon a theory that at the moment when, during the cooling, paraffin wax starts to crystallize out, the latent heat evolved in the crystallization process will retard the cooling of the hydrocarbon liquid. Thus, the cooling curve will show a substantially constant temperature plateau at the point where crystallization of the paraffin wax begins, and the temperature corresponding to this plateau is, at least approximately, the cloud point temperature of the hydrocarbon liquid.
The present invention utilizes the cloud point temperature as the beginning point for determining the presence or absence of a pour point depressant in the hydrocarbon liquid sample. Thus, the present invention relates to first determining the beginning point of the plateau in a temperature versus time cooling curve of a sample and then subsequently determining the ending point for the plateau. It has been discovered that the time interval between the beginning and ending points of a hydrocarbon liquid cooling curve establishes the presence or absence of a pour point depressant additive in the sample liquid when compared to a reference time interval. It has thus been discovered that a reference time interval of about 3.5 minutes is valid over a substantially wide range of cooling rates (e.g. 6.degree. C./hr. to 20.degree. C./hr.) and thus if a measured time interval for the temperature plateau on a cooling rate curve for a hydrocarbon liquid sample is less than or greater than the 3.5 minute reference time interval, the presence or absence respectively, of a pour point depressant additive is confirmed.
Accordingly, the present invention contemplates that a sample of a hydrocarbon liquid is cooled from a temperature substantially above the cloud point temperature to a temperature substantially below the cloud point. The slope of the cooling rate curve is monitored and the points at which a deflection in the curve begins and ends is monitored. The beginning and ending deflections are thus indicative of the beginning and ending of a plateau on the temperature versus time cooling curve of the sample. The time interval between the beginning and ending deflections of the curve are thus determined and this determined time interval is compared to a reference time interval so as to establish the presence or absence of a pour point depressant additive.
Accordingly, the present invention provides a method and apparatus whereby the presence or absence of a pour point depressant additive can be readily determined in a sample hydrocarbon liquid.